Typically, the idea is that I would like drivers behind me to not pass me in the same lane that I'm in — that is, I'm trying to avoid having a driver do the typical half-assed job of passing by trying to squeeze their SUV in the same lane as me. (i.e., if you want to pass me, use another lane, not this one. If there isn't another lane: tough, I'll be out of your way ASAP — I don't like having a car behind me any more than a driver likes having a biker in front of them.)
> Wouldn't this slow the traffic.
Sometimes, yes. For me, most of the routes I take the car traffic moves at approximately the same speed or slower. If there's a huge speed difference and traffic is much faster, I'll usually avoid left turns and use a crosswalk to "turn", if I think that's safer. (Which has its own tradeoffs: one has to be very careful and ensure that drivers in the lane next to you are going to actually stop at the crosswalk before you enter it. Just because the light is red hardly means anything, and a lot of drivers will overshoot the stop line if they don't think anyone is going to be in the crosswalk when they do so.)